YOKOTA AIR BASE, Japan—U.S. Defense Secretary Chuck Hagel arrived in Japan on Saturday on a six-day mission to reassure America's Asian allies that the cautious international response to Russia's intervention in Ukraine shouldn't be viewed as a sign that the U.S. is reluctant to support its friends in times of crisis.

"Allies are going to look at each other to be reassured," he told reporters on board his military plane en route to Japan. "There's no indication or weakness on the part of the United States...